Potential role of marine species-derived bioactive agents in the management of SARS-CoV-2 infection

COVID-19, caused by the SARS-CoV-2 outbreak, has resulted in a massive global health crisis. Bioactive molecules extracted or synthesized using starting material obtained from marine species, including griffithsin, plitidepsin and fingolimod are in clinical trials to evaluate their anti-SARS-CoV-2 and anti-HIV efficacies. The current review highlights the anti-SARS-CoV-2 potential of marine-derived phytochemicals explored using in silico, in vitro and in vivo models. The current literature suggests that these molecules have the potential to bind with various key drug targets of SARS-CoV-2. In addition, many of these agents have anti-inflammatory and immunomodulatory potentials and thus could play a role in the attenuation of COVID-19 complications. Overall, these agents may play a role in the management of COVID-19, but further preclinical and clinical studies are still required to establish their role in the mitigation of the current viral pandemic.

the SARS-CoV-2 Mpro enzyme. Data from this study highlighted the high binding affinity and low flexibility of T3, which can act as an excellent candidate against SARS-CoV-2 [22]. Plitidepsin (Aplidin R ) is a marine-derived (Aplidium albicans) drug with anticancer and antiviral properties. A recent in vitro study conducted by White et al. showed the potent anti-SARS-CoV-2 potential of plitidepsin in two cell lines. It was found to be more potent than remdesivir in inhibiting viral replication in a human cell line (hACE2-293T) [23]. Plitidepsin has also been tested in Phase I/II clinical trials to test its efficacy in COVID-19 patients. Although the results are not yet published, it is suggested by the manufacturer that this agent should be strongly considered for expanded clinical trials to treat COVID-19 patients [23]. In addition to these, other studies have reported the therapeutic potential of natural products, including marine-derived compounds, against SARS-CoV-2 infection [2,24,25]. Data collected from web show that, from 1969 to 2020, a total of 15 marine-derived drugs were approved by the US FDA for the treatment of various chronic disorders including cancers, severe pain and herpes simplex viral infections [26]. Table 1 lists the pharmacological attributes of selected bioactive molecules obtained from different marine species.

Search methodology
Relevant information was collected from review articles, web contents, clinical reports and research papers available at numerous databases including PubMed, ScienceDirect, SciFinder, Google Scholar, Clinicaltrails.gov, ProQuest, Medline and Scopus, respectively. Moreover, the global marine pharmaceutical pipeline website (https://www.mari nepharmacology.org/) maintained by Alejandro MS Mayer at Midwestern University, IL, USA was also consulted to track the current status of marine species-derived bioactive molecules [51]. To locate authentic information from published records, the following combination of terms were used: 'coronavirus and natural drugs', 'microbial metabolites', 'marine by-products against SARS-CoV-2', 'synthetic products of marine origin', 'marine drugs', 'sea-life compounds', 'marine reservoirs having therapeutic potential', 'SARS-CoV-2 and marine species-derived agents' and 'natural products and COVID-19'. The studies encompassing information relevant to crude extracts, fractions, isolated compounds and secondary metabolites obtained from marine species and acting against SARS-CoV-2 were included. All other information not fulfilling the inclusion criteria, duplicate data and having approved antiviral effects other than COVID-19, were excluded.

Results & discussion
Medicinal agents obtained from marine organisms are known to have antiviral activities against enveloped viruses, including coronaviruses (CoVs) [52]. Marine sourced drugs may get access to the market through sustainable availability of an adequate number of organisms without affecting the marine environment [53]. Recently, the USA and Japan have launched new programs for the successful cultivation of sea-life (bacteria, fungi and so on) which are now becoming a potential source of novel and life-saving bioactive agents [54] with profound anti-inflammatory, anticancer and antimicrobial properties. In addition, the development of a universal expression system for the synthesis of small molecules and constructing biogenetic tools for the in vivo evaluation of cultured marine microorganisms are also basic requirements for this research. This review highlights the key findings of various scientific studies conducted to explore the antiviral activities (especially anti-SARS-CoV-2) of medicinal products obtained from different marine organisms (bacteria, cyanobacteria, fungi, algae and invertebrates).

Alkaloids
Quimque and colleagues used molecular docking and MD simulation tools to predict the binding affinity of two marine alkaloids obtained from fungi i.e., scedapin C and norquinadoline A, against major targets of SARS-CoV-2. Both compounds showed the highest binding affinity against PLpro with binding energy values of -10.9 kcal/mol [55]. PLpro has been known to play a major role in SARS-CoV-2 replication within host cells and the deactivation of host immune responses by its interaction with ubiquitin protein [56]. Ubiquitin protein is associated with the activation of NF-κβ signaling with subsequent production of IFN-β in the host [57]. IFN-β is known to have antiviral properties against many viruses, including SARS-CoV-2 [58]. Upon entry of SARS-CoV-2 into the host cell, it releases PLpro, which serves two functions: first, these proteases help in the replication of viruses in the host cell [59], and secondly, they deactivate host immune responses by suppressing NF-κβ activation and subsequently inhibit IFN-β production [57]. Therefore, based on the findings of this molecular simulation study it can be proposed that scedapin C and norquinadoline A alkaloids have ability to inhibit SARS-CoV-2 PLpro, thus blocking viral replication with simultaneous activation of host immune responses through NF-κβ signaling pathway. Further detailed in vitro and in vivo studies are warranted in this regard to support this hypothesis. future science group www.futuremedicine.com It can reconstitute the gut microbiota, reduce bacterial metabolite-driven peripheral infiltration of immune cells into the brain, and inhibit neuro-inflammation in the brain as observed in animal models. Some GV-971 can penetrate the brain and directly inhibit A␤ fibril formation and destabilizes the preformed fibrils into nontoxic monomers [42] ET-743 Ecteinascidia turbinate (colonial tunicate) Antitumor activity in patients with advanced resistant sarcoma in Phase III clinical trial (identifier: NCT02903004) 1. Induces a broad inhibition of activated transcription with no effect on the constitutive transcription. 2. ET-743 inhibits the activation of the multidrug-resistant pathway. [43] Only datum of clinical trials which were completed or in the phases of recruitment is included. OC: Ovarian cancer; VGSC: Voltage-gated sodium channel; VZV: Varicella Zoster virus. A recent study conducted by Khan and colleagues demonstrated the efficacy of five marine alkaloids i.e., C1 (fostularin-3) and C2 (chimyl alcohol) obtained from Aplysinidae family, C3 (palmitoleic acid) obtained from Pterogorgia citrine, and C4 (cannabigerolic acid) and C5 (acitretin) obtained from Petrosia strongylophora sp. against SARS-CoV-2. Data of molecular docking and MD simulation techniques exhibited that fostularin-3 (C1) formed hydrogen bonds and hydrophobic interactions with Thr24, Leu27, His41, Phe140, Cys145, His163, Met165, Pro168 and His172 residues in the active site of the M pro enzyme of SARS-CoV2 [22].

Review
Caulerpin is another alkaloid obtained from different species of marine algae including green macroalgae (Caulerpa racemose), red algae (Chondria armata), and brown algae (Sargassum platycarpum). Abdelrheem and colleagues used molecular docking and MD simulation techniques to predict the effects of caulerpin against the main protease of SARS-CoV-2 (3CLpro) and its pharmacokinetic properties. The results of the molecular docking study indicated the strong binding affinity of caulerpin with 3CLpro. Moreover, it was found to form a stable complex with 3CLpro in explicit water. This complex had no major effect on the flexibility of the protein throughout the simulations, making it a suitable candidate in COVID-19 treatment [60]. Besides this, caulerpin has been reported to have well-known potential against pathogenic viral strains like HSV-1 and hRSV [61]. 3CLpro is known to play a major role in the maturation of virus [60] so keeping in mind the data of the above mentioned computer simulation study, it can be concluded that caulerpin has the potential to halt the SARS-CoV-2 life cycle. Various studies have reported the in vivo anti-inflammatory properties of caulerpin. Data from these studies revealed that it produces anti-inflammatory effects through the down-regulation of TNF-α, IFN-γ, IL-6 and IL-17 [62,63]. Studies have shown that TNF-α and IL-6 are major culprits secreted in excess during cytokine storm phase (hyperinflammatory responses) of COVID-19 and are responsible for organ damage [64,65]. IL-6 is known to stimulate the secretion of IL-17, another pro-inflammatory cytokine, from Th17 cells via the JAK2/STAT3 pathway. On the basis of these studies, it is proposed that caulerpin, which has favorable pharmacokinetic (ADMET) properties, has the potential to not only block SARS-CoV-2 3CLpro actions but also can attenuate inflammation future science group www.futuremedicine.com via the down-regulation of multiple pro-inflammatory cytokines released during the hyperinflammatory phase of COVID-19.
C-phycocyanin C-phycocyanin is a protein-bound pigment of the blue-green algae (Arthrospira platensis), commonly known as spirulina. Raj and colleagues screened its binding affinity with the non-structural proteins (nsp-8, nsp-7 and nsp-12) of SARS-CoV-2. These proteins, particularly nsp-12, are known to play a major role in SARS-CoV-2 multiplication in the host cells. Docking data revealed that C-phycocyanin has the potential to inhibit the active sites of nsp-12 [66]. Moreover, spirulina is used as a food supplement due to its rich nutritional composition including vitamin C and minerals. Studies have suggested that elderly people should consume spirulina in order to boost their immunity against SARS-CoV-2 [67]. Moreover, spirulina is known to have anti-inflammatory properties in vivo through the inhibition of inflammatory cells recruitment and down-regulation of TNF-α and nitric oxide release [68]. Keeping in view the demonstrated pharmacological properties of spirulina, it is urged that it should be studied for its effectiveness in the management of various phases of COVID-19 using well-planned preclinical and clinical studies.

Polyphenols
Studies have shown that various algae polyphenols (quercetin and its analogues) derived from the brown macro algae of genus Sargassum, possess potent antiviral properties [69]. Data obtained from a scientific report showed that quercetin plays a significant role in the deactivation of interaction sites of SARS-CoV-2 proteins that attack human ACE2 receptors in the lungs. Quercetin showed low binding energy and extraordinary potency for the targeted proteins, and thus can be considered as a potential agent in the development of anti-SARS-CoV-2 therapy. However, further animal studies are suggested to confirm the efficacy of quercetin as an anti-CoV therapy [14]. SARS-CoV-2 M pro is an enzyme with a dominant function in viral replication and translation thus, can be considered an attractive target site for anti-CoV drugs [70]. Based on the extensive ongoing investigations on marine-derived flavonoids, Antonio and colleagues studied the M pro inhibitory effects of quercetin, apigenin, luteolin and amentoflavone (bioflavonoids) in in vitro assays. They revealed that poly-hydroxylated mixtures impart a prominent role in the development of M pro noncompetitive inhibitors, and exhibit marked potential against SARS-CoV-2 reproduction. A few studies have shown that quercetin-based nebulization alleviates respiratory symptoms [71]. Recruitment of monocytes and macrophages in COVID-19 leads to lung inflammation and pulmonary fibrosis [72]. Hence, quercetin isolated from an algal (Acanthophora spicifera) species was tested extensively for its pharmacological actions against respiratory pathology [73]. Multiple studies have shown that almost all COVID-19-infected patients have lungs abnormalities due to the release of inflammatory mediators, thus, it is suggested that quercetin may serve as an attractive alternative therapy for inflammation-associated respiratory ailments, including COVID-19 [74]. Several in vivo and ex vivo studies have reported the virucidal effects of quercetin via interruption of the entry and attachment phases of the viral life cycle (e.g., HCV, herpes simplex virus, hepatitis B virus, vesicular stomatitis virus, adenovirus and influenza-A virus). Molecular docking techniques and ex vivo as well as in vivo models have provided the basis to explore quercetin's potential against SARS-CoV-2 [75]. Findings of the recent pilot randomized controlled trial showed that treatment of COVID-19 patients with Quercetin Phytosome R resulted in the faster recovery and marked reduction in blood parameters i.e., LDH (-35.5%), Ferritin (-40%), CRP (-54.8%) and D-dimer (-11.9%) [76]. However, the emergence of high-quality clinical data is encouraged to support the possible efficacy of quercetin in the context of the recent viral pandemic (SARS-CoV-2) [77].

Sulfated polysaccharides
Song and colleagues studied the in vitro inhibitory activities of four marine sulfated polysaccharides, i.e., sea cucumber sulfated polysaccharide (SCSP), fucoidan from brown algae, iota-carrageenan from red algae and chondroitin sulfate C from sharks (CS) against SARS-CoV-2. Among these polysaccharides, SCSP, fucoidan and carrageenan showed significant anti-SARS-COV-2 activities at the concentrations of 3.90-500 μg/ml. SCSP exhibited the strongest inhibitory activity among these compounds. Further experimentation using pseudo-type virus bearing the S-glycoprotein showed that SCSP can bind to the S-glycoprotein of SARS-CoV-2 thus, indicating the potential to prevent SARS-CoV-2 entry into the host cells. Based on the findings of this in vitro study, it was suggested that sulfated polysaccharides obtained from different marine organisms can be employed to treat and prevent COVID-19 [78].
Mechanical ventilation with oxygen is widely used to treat various lung illnesses; however, it may result in hyperoxia, causing lung inflammation and injury [79]. In vivo studies conducted by Nie and colleagues showed that treatment with fucoidan prevented lung injury by reducing hyperoxia-induced inflammation in mice and suppressing the expression of pro-inflammatory cytokines (IL-1, IL-16 and TNF-α) via the ERK signaling pathway [79]. A review by Fitton and colleagues highlighted that fucoidan has immune boosting properties. Moreover, it has potent vaccine adjunct properties [80] and studies conducted by Negishi et al. in elderly Japanese patients showed that oral administration of a fucoidan extract enhanced their responses to influenza vaccines [81]. Taken together, these findings highlight that fucoidan not only has the potential to target SARS-CoV-2, but can also produce beneficial effects in humans by increasing their responsiveness to the COVID-19 vaccines as well as reducing serum levels of pro-inflammatory cytokines. However, further pre-and clinical studies in this regard are warranted.
Briarane-type diterpene A molecular docking study calculated the binding potency of natural products obtained from a variety of sources including marine organisms against major drug targets of SARS-CoV-2. The data showed that excavatolide M, a briarane-type diterpene obtained from gorgonian (Briareum excavatum), has the potential to bind and block SARS-CoV-2 TMPRSS2 (S-protein) [14]. However, another docking study conducted by Rahman and colleagues predicted the toxic nature of excavatolide M [82]. Further studies to explore the pharmacological and toxicological profile of excavatolide M are required to establish its safe dose and exact mechanism of action.

Sesquiterpene quinone
A study by Surti and colleagues evaluated the potency of illimaquinone, a marine sponge metabolite first identified and isolated from brown sponge named Hippospongia metachromia, against SARS-CoV-2 using in silico approach broadly. Efficacy of ilimaquinone against nine potential SARS-CoV-2 target proteins was calculated and compared with hydroxychloroquine, azithromycin, favipiravir, ivermectin and remdesivir using molecular docking studies. Binding energies data showed that ilimaquinone hold promising inhibitory potential against all the SARS-CoV-2 target proteins. The study concluded that ilimaquinone can be the most promising inhibitory candidate against the SARS-CoV-2 papain like protease [83].

Fingolimod
Fingolimod is a synthetic compound that was synthesized from myriocin, a metabolite of the fungus called saria sinclairii. The Australian and American drug regulatory authorities have approved the use of fingolimod in relapsingremitting multiple sclerosis, an autoimmune disease [84]. Data on the use of fingolimod in COVID-19 patients is still not clear. One group of scientists suggests that it could be beneficial in COVID-19 patients as it can arrest the invasion of lymphocytes into alveoli [85], which could be beneficial as further lung damage can be prevented. Another group of scientists reported clinical improvement in COVID-19 patients with multiple sclerosis [86]. Moreover, a third group of scientists reported worsening of SARS-CoV-2 infection in multiple sclerosis patients upon the discontinuation of fingolimod therapy [87]. A clinical trial registered at https://clinicaltrials.gov (identifier: NCT04280588) is on the way (Phase II) to establish the efficacy of fingolimod in COVID-19 patients.

Seaweed lectins
Seaweed lectins, especially highly mannose-specific lectins from red algae including griffithsin derived from Griffithsia sp., are known to block the replication of various enveloped viruses like HIV-1, influenza and herpes viruses in in vitro assays. In 2010, O'Keefe and colleagues reported that griffithsin binds to the SARS-CoV spike glycoprotein and inhibits viral entry into host cells. Moreover, it prevented damage to host cells caused by the abnormal activation of the host immune system during the infection [89]. Millet et al. in 2016 reported the antiviral activity of griffithsin against another strain of coronaviruses, MERS-CoV. It was reported to inhibit the entry of MERS-CoV future science group www.futuremedicine.com Table 2. Mechanisms, specific functions and characteristics of selected marine-derived natural products against severe acute respiratory syndrome coronavirus.

Drugs Source Mechanism of action Specific characteristics
Ref.

Alpysinidae
Have the potential to bind with residues of active sites of M pro of SARS-CoV-2 Antiviral and protease inhibitor [22] Iota-carrageenan Red algae Binds with S-glycoprotein of SARS-CoV-2 and stops the entry of virus in the host cell Antiviral and inhibitors of proliferative proteins [60] Fucoidan Brown algae Blocks ERK signaling pathway and inhibits S-glycoprotein of SARS-CoV-2 Antiviral, immunomodulatory and anti-inflammatory [78] Palmitoleic acid Pterogorgia citrine Blocks M pro activation and counter endothelial dysfunctioning to inhibit inflammatory pathways Antiviral, anti-inflammatory and protease inhibitor [22] Cannabigerolic acid and Acitretin

Petrosia strongylophora
Targets essential proteins involved in the lifecycle of SARS-CoV-2 through deactivation of active sites of M pro COX-enzyme inhibitor and antiviral [22] C-phycocyanin Arthrospira platensis (blue-green algae) Acts on active sites of nsp-12 and inhibit the multiplication of SARS-CoV-2 Immunomodulatory, anti-inflammatory, antiviral and nutritional supplement [67,68] Caulerpin

Caulerpa racemosa
Have significant binding affinity with 3CLpro to stop SARS-CoV-2 activation and attenuate inflammation via down-regulation of IL-6, TNF-␣ etc.
into host cells by blocking its spike proteins [90]. Barre and colleagues conducted a detailed in silico docking study to explore the potential anti-SARS-CoV-2 properties of griffithsin. It was proposed to hamper the attachment of SARS-CoV-2 to host cells by making a complex with S-proteins [25]. Data from https://clinicaltrials.gov (identifiers: NCT02875119 and NCT04032717) show that two clinical trials are registered to evaluate the efficacy of griffithsin in HIV infections. This indicates that this glycoprotein has the potential to be developed as an antiviral or an anti-SARS-CoV-2 nutraceutical. Table 2 highlights the summary of selected marine bioactive molecules against COVID-19.

Miscellaneous agents
Gentile and colleagues used in silico docking tools to explore the anti-SARS-CoV-2 potential of natural products obtained from different marine sources. Out of 770 compounds that met the pharmacophore criteria, 17 compounds including heptafuhalol A, phlorethopentafuhalol A and B, pseudopentafuhalol C, pseudotheonamide C and D, hydroxypentafuhalol A, pentaphlorethol B, 6,6'-Bieckol, 8,8'-Bieckol, apigenin-7-oneohesperidoside, luteolin-7rutinoside, dieckol, aeruginosin 98B, resinoside B, pentaphlorethol A and tunichrome An2 were found to have promising SARS-CoV-2 protease inhibitory potentials [92]. Further in vitro and in vivo studies to establish the possible role of these agents in the prevention and treatment of SARS-CoV-2 infections and associated complications are warranted.

Conclusion
In the current health emergency, COVID-19 has become a global issue affecting the daily routine of millions of people around the globe. Data obtained from currently available studies point towards promising anti-SARS-CoV-2 potential of medicinal agents obtained from various marine organisms. Some of these like spirulina are already in human use as a food supplement and other like fingolimod, griffithsin and plitidepsin are under clinical trials to evaluate their anti-SARS-CoV-2 and anti-HIV efficacies.

Limitations & future perspective
Currently available studies on the use of marine-derived medicinal agents as anti-SARS-CoV-2 drugs are exceptionally preliminary and majority of the information available is based on computer-aided findings, for example, using molecular docking and MDs simulation techniques. Detailed in vitro and in vivo studies utilizing multiple animal models are suggested to gather conclusive evidences before the commencement of clinical studies. A collaborative approach involving marine biologists, chemists, biotechnologists and pharmacists could facilitate and speed up preclinical screening. Obtaining a continuous and consistent supply of sufficient amounts of organisms and compounds without harming the marine environment is another crucial step in the marine drug development process. Possible approaches to counteract this supply problem are the use of biotechnology techniques (aquaculture/fermenter cultivation; genetic engineering; enzymatic synthesis or modification) and by the chemical synthesis/semi-synthesis/modification of isolated bioactive molecules.

Alkaloids
• Alkaloids obtained from different marine sources have the capacity to bind with and inhibit SARS-CoV-2 main viral protease (M pro ).

Polyphenols
• Docking studies showed that polyphenols play a major role in the deactivation of interaction sites of SARS-CoV-2 proteins that attack human ACE2 receptors in the lung. Spirulina • A protein-bound pigment of the blue-green algae is commonly used as a food supplement and has immune boosting, anti-inflammatory and SARS-CoV-2 nsp binding properties.

Marine medicinal agents in clinical trials
• Griffithsin and plitidepsin have shown promising results in preliminary clinical trials and are actively being investigated for their anti-SARS-CoV-2 potentials.

Author contributions
All the authors contributed to the acquisition of data, and critical review and approved the final version of this manuscript.